The trend to 3D and heterogeneous integration enable driving multi-functional blocks in one package. Flip-chip integration is currently playing an important role and is based on solder joints. To overcome the limitations of solder joints, all-copper interconnects have been investigated to meet electrical, thermal, and reliability demands in 3D integration. The underfill process is widely applied in flip-chip encapsulation technology. We propose a novel wafer-scale all-Cu interconnect method combining epoxy-based photo-patternable polymer as self-aligned underfill layer with the patterned copper nanoparticles interconnects. The resulting test wafers were able to pattern 20 µm pitch copper nanoparticle-paste interconnects on both substrates with and without photoimageable polymer. The Cu paste was applied to form the interconnects and was sintered after bonding process. Free-standing nanocopper is sintered to obtain mechanical properties with a Young's modulus of 112 GPa. All-Cu interconnects with diameter of 50 µm and 100 µm were measured to achieve the specific contact resistance, ranging from 1.4 × 10-5O· cm2 to 1.0 × 10-5O· cm2 at different sintering temperature when epoxy-based underfill existing. And its resistivity was 4.54× 10-4 O· cm, compared to 5.86× 10-4O· cn for the all-Cu interconnects without underfill.